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Abdelbaset al-Megrahi was the only man to be convicted of the 1988 atrocity

Six miles above Scotland the jumbo jet's crew were preparing to serve dinner when the bomb exploded.

Pan Am flight 103 was about 38 minutes into its journey from London Heathrow to New York's John F Kennedy Airport when it was ripped apart over Dumfriesshire.

Many of the 259 people on board the Boeing 747 were heading home to the United States for Christmas.

In Lockerbie they do not dwell on the memories of that night: the sky lighting up, the "rushing and screaming noise", the "liquid fire" which rained down, killing 11 residents of the small market town.

Britain and America alleged that these horrors were the work of a Libyan intelligence agent, Abdelbaset al-Megrahi.

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On the ground in Lockerbie 11 people were killed

The trial of Megrahi and his fellow countryman, Al Amin Khalifa Fahima, was the result of the biggest criminal investigation in British history, led by the country's smallest police force, Dumfries and Galloway Constabulary.

But almost from the start the inquiry was dogged by claims of a conspiracy and, even now, key questions about the bombing remain unanswered.

Libya was not the first suspect.

Early evidence pointed to a Palestinian terrorist group, the Popular Front for the Liberation of Palestine-General Command (PFLP-GC), allegedly working on behalf of Iran.

'Iranian revenge'

The motive was said to be retaliation for the shooting down of an Iranian passenger jet by an American warship in July 1988 with the loss of 290 lives.

In October 1988, two months before the bombing, West German police raided flats near Frankfurt where the PFLP-GC was preparing bombs in Toshiba radio cassette players, similar to the one used in the Lockerbie bombing.

But it was forensic science in Scotland which cast doubt on the theory of Iranian revenge and ensured that suspicion instead fell on Libya.

From the Solway Firth to the North Sea, the bombing left debris strewn over 845 square miles - yet the key piece of evidence was a fragment of circuit board no bigger than a fingernail.

The circumstances of its discovery and its subsequent handling are controversial, but experts said it was part of the timer for the bomb and was wrapped in a scrap of clothing from a shop on Malta, the Mediterranean island between Italy and Libya.

In November 1991 warrants were issued for the arrest of Megrahi, the head of airline security for Libyan Arab Airlines (LAA) and Fahima, the airline's station manager at Luqa Airport, Malta.

Diplomatic wrangling

They were accused of using their positions with LAA to load a bomb onto a plane at Luqa Airport.

The explosives were apparently hidden inside a Toshiba radio cassette player, wrapped in clothing and packed in a brown Samsonite suitcase.

The case was then said to have been flown unaccompanied via Frankfurt to Heathrow and loaded on board Pan Am flight 103 to New York.

We do not know what the motivation for the bombing was, who ordered it, why it was carried out, how it was allowed to happenPamela Dix, Lockerbie bombing victim's sister

After a decade of diplomatic wrangling, the suspects made the journey from Tripoli to a special Scottish court at Camp Zeist in the Netherlands.

Uniquely for a modern Scottish murder trial their fate would be decided not by a jury but by three judges: Lord Sutherland, Lord Coulsfield and Lord Maclean.

On 31 January 2001 they announced their verdict, acquitting Fahima but finding Megrahi guilty of 270 counts of murder and sentencing him to life in prison.

But doubts about the conviction linger.

Some observers claim it suited the West to frame Megrahi - and Libya - because the help of the prime suspect Iran and its ally Syria had become essential in the first Gulf War against Iraq.

Doubts about the evidence were even expressed by the three trial judges, who recognised "a number of uncertainties and qualifications" in the prosecution case.

In their 82-page judgment they identified three "important witnesses" for the Crown: Abdul Majid, a Libyan who spied for the US in Malta; Edwin Bollier, whose Swiss firm was alleged to have made the timer for the bomb; and Tony Gauci, a Maltese shopkeeper who claimed he had sold Megrahi the clothing which was wrapped around the device.

The judges described both Mr Majid and Mr Bollier as unreliable, adding that there were "undoubtedly problems" with the identification of Megrahi as the man who bought the clothing.

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Scots Justice Secretary Kenny MacAskill said the decision to release Megrahi was the right one

Furthermore "the absence of an explanation as to how the suitcase was taken into the system at Luqa" was "a major difficulty for the Crown case".

Nonetheless, they concluded that "...the conception, planning and execution of the plot which led to the planting of the explosive device was of Libyan origin" and there was a "real and convincing pattern" of evidence which proved Megrahi's guilt.

But the judicial qualifications were seized on by his lawyers, who began an appeal.

It was dismissed by a panel of five judges in March 2002. Supporters of the verdict pointed out that no fewer than eight Scottish judges had now validated the conviction: surely no conspiracy could be so broad?

And yet the controversy did not abate.

Within months, the former South African president, Nelson Mandela, visited Megrahi in Glasgow's Barlinnie prison, telling reporters: "It will be a pity if no court reviews the case."

Five years later, the Scottish Criminal Cases Review Commission ruled that there were six grounds for a second appeal.

The commission's full report has not been officially released but details emerged when it was published online by a newspaper.

Although it rejected concerns about the forensic evidence linking Megrahi with Malta, it did question other evidence which placed him on the island.

It concluded that evidence which cast doubt on Mr Gauci's identification of Megrahi as the purchaser had not been made available to the defence, a breach of rules designed to ensure a fair trial.

Triumphant scenes

In particular, there was evidence that four days before he identified Megrahi, Mr Gauci had seen a photograph of him in a magazine article about the bombing.

Mr Gauci was also said to have been paid a reward - perhaps $2m - for his assistance with the investigation.

Several appeal hearings had already been held when Megrahi, now diagnosed with terminal prostate cancer, abruptly abandoned proceedings shortly before he was granted compassionate released from jail on 20 August 2009 to return home to Libya.

The Scottish government insisted the decision to let him go was based solely on medical advice that he had around three months to live, not on political considerations, and denied that a deal had been done to free him if he dropped his appeal.

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The scenes which greeted Megrahi's return to Libya caused revulsion in the UK and US

But triumphant scenes in Tripoli, where Megrahi was greeted by cheering crowds waving Scottish saltires, caused revulsion in the UK and US.

Even worse for the Scottish government's credibility was the inaccuracy of the prognosis.

The ensuing row damaged relations between Edinburgh and Washington but the Scottish Justice Secretary Kenny MacAskill continues to defend his decision to free a convicted mass murderer after 10 years in custody.

Giving evidence to MPs he insisted he had no regrets, saying: "We followed the rules and guidance. We believe that we came to the right decision for the right reasons."

But Megrahi's release added more questions to a long list.

Pamela Dix, who lost her brother Peter in the bombing, continues to campaign for a public inquiry.

I have a horror of the idea of my daughter's murder being set in a bag of lies and false evidence that was led against this manJim Swire, Victim's father

"We do not know what the motivation for the bombing was, who ordered it, why it was carried out, how it was allowed to happen with the amount of information that the intelligence services had at that time concerning threats against American aviation," she said.

And she points out that even those convinced of Megrahi's guilt accept that he could not have acted alone.

They include Peter Lowenstein, whose 21-year-old son Alexander died in the bombing.

He believes Megrahi was acting on orders from Libya's leader Colonel Muammar Gaddafi.

He told the BBC: "Am I sad that Megrahi is dead? No.

"There is no doubt in my mind ...that Mr Megrahi was the one responsible for planting the bomb."

Dr Jim Swire, whose 23-year-old daughter Flora was one of the victims, disagrees.

"I'm not claiming at all that the Gaddafi regime is totally innocent in this," he said, "simply that their man Megrahi was not guilty as charged.

"I have a horror of the idea of my daughter's murder being set in a bag of lies and false evidence that was led against this man. I want it to be set in terms of the truth. I want people to know why she died."

There is now a glimmer of hope in this search for the truth.

With Colonel Gaddafi ousted, campaigners for justice hope that information will emerge from Tripoli which sheds light on the bombing.

Senior Scottish and American officials visited the Libyan capital last month in an attempt to pave the way for a renewed inquiry there by Dumfries and Galloway detectives.

Will it confirm Megrahi's guilt and implicate his masters?

Wealth of evidence

Or will it emerge that Libya was not to blame, but regarded Megrahi's conviction - along with an admission of responsibility and compensation payments to Lockerbie relatives - as a price worth paying for the lifting of sanctions and the striking of oil deals in the desert?

In short, was Megrahi a pawn, sacrificed to bring his country in from the cold?

Scottish prosecutors vehemently argue that he was not. They continue to insist that the trial was fair and that Megrahi was convicted because a wealth of circumstantial evidence proved his guilt beyond reasonable doubt.

But more than two decades after death rained down on a small Scottish town, everyone agrees on one thing: someone, somewhere has escaped justice for the Lockerbie bombing.